1. Field of the Invention
The present invention relates to a signal transmitting and receiving apparatus and the method thereof, especially to a signal transmitting and receiving apparatus capable of preventing the receiving end from saturating and the corresponding method.
2. Description of the Prior Art
FIG. 1 illustrates an application circuit for Radio Frequency Identification (RFID) technology. In FIG. 1, a reader 100 includes a receiving circuit 130, a demodulator 140, a processing circuit 150, a modulator 160, an oscillator 170 and a power amplifier 180. The receiving circuit 130 receives RF signal through the antenna 110 and the power amplifier 180 emits RF signal through the antenna 120. When the reader 100 is about to emit an RF signal to the RFID tag 200, the modulator 160 modulates an oscillating signal generated by the oscillator 170 according to the control signal of the processing circuit 150 to form a modulated signal and the modulated signal is then amplified by the power amplifier 180 before being emitted through the antenna 120. The RFID tag 200 receives the RF signal RF1 from the antenna 120 and reflects an RF signal RF2 to the reader 100 after processing the signal RF1. The reflection RF signal RF2 is transmitted to the antenna 110 and then received by the receiving circuit 130. After being demodulated by the demodulator 140, the signal RF2 is further processed by the processing circuit 150. Ideally, the RF signal emitted through the antenna 120 will be transmitted to only the RFID tag 200; however in practice, a portion of the leakage RF signal, RF3, will be received by the antenna 110. Generally, signals amplified by the power amplifier 180 possess larger power (e.g., 30 dBm). Therefore, if the antenna isolation is not good enough (e.g., 25 dB to 30 dB), the receiving circuit 130 will probably saturate upon receiving the high-power leakage signal RF3. As a result, the reader 100 fails since the receiving circuit 130 is not able to receive the reflection signal RF2 having a power of e.g., −80 dBm.